The present invention generally relates to optical semiconductor devices and more particularly to an optical semiconductor device that uses a multilayer reflector.
A multilayer reflector is an optical reflector formed of an alternate repetition of first and second layers having respective refractive indices. As a result of such an alternate repetition of the first and second layers, there appears a periodically changing profile of refractive index in the multilayer reflector, while such a periodically changing refractive index profile causes a Bragg reflection in the optical beam incident thereto with a wavelength that satisfies a condition of Bragg reflection. A multilayer reflector is easily formed on a semiconductor structure by an epitaxial process. Thus, the multilayer reflector is used extensively in so-called vertical-cavity surface-emitting laser diode that emits an optical beam perpendicularly to the epitaxial layers.
In a vertical-cavity surface-emitting laser diode, a multilayer reflector is disposed up and below an active layer in which optical radiation is produced as a result of stimulated emission. Thus, a vertical-cavity surface-emitting laser diode is suitable for integration on a semiconductor substrate in the form of two-dimensional array. In relation to this advantageous feature, an extensive application is expected for a vertical-cavity surface-emitting laser diode as an optical source of various optical telecommunication systems, optical information processing systems or optical interconnection switches.
A laser diode for use in optical telecommunication or optical information processing is generally designed to produce an output optical beam in the 1.3 .mu.m or 1.5 .mu.m wavelength band, in view of the optical transmission band of the optical fiber used in such conventional optical telecommunication or optical information processing systems as an optical transmission medium. In relation to the foregoing specific wavelength band, the conventional laser diodes for use in optical telecommunication or optical information processing have used GaInPAs for the active layer. Further, in relation to the use of the GaInPAs active layer, the conventional laser diodes, including the vertical-cavity surface-emitting laser diodes, have used InP for the substrate. Thereby, the GaInPAs active layer has a composition such that a lattice matching is achieved to the InP substrate while simultaneously having a bandgap energy corresponding to the foregoing optical wavelength band of 1.3 .mu.m or 1.5 .mu.m. Alternatively, the GaInPAs active layer has a composition so as to accumulate a strain therein. In the latter case, the active layer has to be formed to have a thickness not exceeding a critical thickness of the strained heteroepitaxial system formed of the GaInPAs active layer and the InP substrate. When the thickness of the GaInPAs active layer has exceeded the critical thickness, an extensive formation of lattice misfit dislocations would occur in the GaInPAs active layer.
In the foregoing vertical-cavity surface-emitting laser diode for telecommunication applications, a first multilayer reflector is provided between the InP substrate and the GaInPAs active layer, while a second multilayer reflector is provided on the top part of the device. For example, the second multilayer reflector may be provided on a cladding layer covering the GaInPAs active layer.
The multilayer reflector used in the conventional 1.3 .mu.m or 1.5 .mu.m band vertical-cavity surface-emitting laser diode is typically formed of an alternate repetition of a first epitaxial layer of InP and a second epitaxial layer of GaInPAs in view of the need of maintaining a lattice matching to the InP substrate. However, the heteroepitaxial system of GaInPAs and InP has a drawback in that the change of the refractive index is very small. In terms of the refractive index difference between the first epitaxial layer and the second epitaxial layer, the change is only in the order of about 0.25. Thus, in order that the multilayer reflector is effective, it has been necessary to increase the number of stacks of the first and second epitaxial layers in the multilayer reflector. For example, it has been necessary to stack the first and second epitaxial layers 40 times or more in order to achieve a desired reflectance of 99.9%. However, such an increased number of stacks of the first and second epitaxial layers increases the time needed for forming the epitaxial structure of the multilayer reflector, and the throughput of production of the laser diode is decreased inevitably. Further, such an increased number of stacks inevitably leads to an increased thickness of the multilayer reflection structure and hence an increased step height. Thereby, the fabrication of the laser diode becomes substantially difficult. For example, a conventional multilayer reflection structure may have a total thickness of as much as about 20 .mu.m, while the multilayer reflection structure having such a very large thickness tends to suffer from the problem of variation in the thickness of the first and second epitaxial layers in the thickness direction. When this occurs, the desired high reflectance is not achieved. In order that the multilayer reflection structure is to be effective, it is necessary that each of the first and second epitaxial layers in the structure has a thickness corresponding to one-quarter (.lambda./4) of the wavelength of the optical beam to be reflected.
In order to eliminate the foregoing problem, the Japanese Laid-Open Patent Publication 6-132605 describes a laser diode that uses a strained buffer layer of GaInPAs formed on an InP substrate. The buffer layer has a composition that induces a lattice misfit to the underlying InP substrate and carries thereon an active layer of GaInPAs that achieves a lattice matching to the buffer layer, with a first multilayer reflection structure and a first cladding layer interposed in this order between the buffer layer and the active layer. The first multilayer reflection structure includes an alternate repetition of a first epitaxial layer of AlInAs and a second epitaxial layer of GaInPAs having a composition of Ga.sub.x1 In.sub.1-x P.sub.y1 As.sub.1-y1 (0.ltoreq.x.sub.1 .ltoreq.1, 0.ltoreq.y1.ltoreq.1) each having a thickness corresponding to a quarter of the wavelength (.lambda./4) of the optical beam to be reflected, wherein the first and second epitaxial layers have respective compositions selected so as to establish a lattice matching to the buffer layer. Further, the first cladding layer is formed of GaInPAs having a composition Ga.sub.x2 Iv.sub.1-x2 P.sub.y2 As.sub.1-y2, which is set such that the first cladding layer establishes a lattice matching to the buffer layer.
The active layer of GaInPAs is provided on the first cladding layer noted above, wherein the active layer has a composition of Ga.sub.x3 Iv.sub.1-x3 P.sub.y3 As.sub.1-y3 (0.ltoreq.x3.ltoreq.1, 0.ltoreq.y3.ltoreq.1) selected such that the active layer establishes a lattice matching to the buffer layer. Further, a second cladding layer of GaInPAs having a composition of Ga.sub.x4 As (0.ltoreq.x4.ltoreq.1, 0.ltoreq.y4.ltoreq.1) is provided on the active layer in lattice matching to the foregoing buffer layer, and a second multilayer reflection structure similar to the first multilayer reflection structure is provided on the second cladding layer. Thus, the second multilayer reflection structure is formed of an alternate repetition of a third epitaxial layer of AlInAs and a fourth epitaxial layer of GaInPAs having a composition of Ga.sub.x5 Iv.sub.1-x5 P.sub.y5 As.sub.1-y5 (0.ltoreq.x5.ltoreq.1, 0.ltoreq.y5.ltoreq.1) both achieving a lattice matching to the buffer layer.
In this prior art laser diode, the epitaxial layers forming the first multilayer reflection structure, the first cladding layer, the active layer, the second cladding layer and the second multilayer reflection structure, all have a composition that achieves a lattice matching to the GaInPAs buffer layer. In other words, all the foregoing layers, including the buffer layer, have a lattice constant smaller than the lattice constant of InP and intermediate to the lattice constant of GaAs. Thereby, it becomes possible to increase the refractive index difference between the first and second epitaxial layers of the first multilayer reflection structure or between the third and fourth epitaxial layers of the second multilayer reflection structure, by using a combination of AlInAs and GaInPAs or AlInP and GaInPAs for the first and second epitaxial layers or for the third and fourth epitaxial layers. As a result, the number of stacks of the first and second epitaxial layers necessary for achieving a desired reflectance for the first or second multilayer reflection structure is reduced substantially.
However, the foregoing conventional laser diode inherently relies upon a strained system constructed on the InP substrate, and the laser diode tends to suffer from the problem of deterioration of device performance due to the creation of misfit dislocations and the like pertinent to a strained heteroepitaxial system. Further, the foregoing prior art laser diode has suffered from a poor high temperature performance due to the relatively small band discontinuity between the active layer and the first or second cladding layer. Because of the small band discontinuity, the carriers in the active layer experience a poor carrier confinement, and the oscillation characteristic of the laser diode deteriorates rapidly with increasing temperature due to the overflowing of the carriers away from the active layer.
In view of the foregoing poor high-temperature performance of the vertical-cavity surface-emitting laser diode, there is also a proposal to form an epitaxial layer of GaInAs on a GaAs substrate for maximizing the band discontinuity in the conduction band. When a GaInAs layer is used, the bandgap energy of GaInAs decreases with increasing In content. On the other hand, the increased In content in such a GaInAs mixed crystal leads to an increased lattice constant and hence increased compressional strain when the mixed crystal layer is grown on a GaAs substrate, while the increased compressional strain in the GaInAs active layer acts to decrease the bandgap thereof further. In this way, the laser oscillation wavelength of the laser diode can be increased up to about 1.1 .mu.m due to the effect of increased In content and the effect of accumulation of the compressional strain, while the foregoing oscillation wavelength of about 1.1 .mu.m is thought to be the maximum wavelength that can be reached according to such an approach. An oscillation wavelength of 1.3 .mu.m or 1.5 .mu.m needed for optical telecommunication or optical information processing is not realized.
On the other hand, the Japanese Laid-Open Patent Publication 6-37355 describes a laser diode that uses a mixed crystal of GaInNAs as an active layer in combination with a substrate of GaAs. According to the teaching of the foregoing prior art, it becomes possible to set the composition of the GaInNAs mixed crystal such that the GaInNAs mixed crystal establishes a lattice matching to the GaAs substrate while simultaneously realizing a laser oscillation at the wavelength of 1.3 .mu.m or 1.5 .mu.m.
FIG. 1 shows the band diagram of various group III-V compound semiconductor materials including the foregoing GaInNAs mixed crystal.
Referring to FIG. 1, it will be noted that there appears a very large negative bowing in the bandgap energy in the mixed crystal system of GaAs-GaN and the bandgap energy of a GaAs mixed crystal decreases with increasing In content. GaN itself has a very large bandgap and is used for an LED or laser diode that emits a blue or violet radiation. Further, by incorporating In with an appropriate amount into a GaInNAs mixed crystal, it is possible to adjust the bandgap energy below the bandgap energy of GaAs while maintaining the lattice constant identical to the lattice constant of GaAs. Thus, by using the GaInNAs mixed crystal for the active layer, it becomes possible to fabricate a laser diode, including the vertical-cavity surface-emitting laser diode, on a GaAs substrate in lattice matching thereto.
In such a vertical-cavity surface-emitting laser diode operable in the 1.3 .mu.m band and constructed on a GaAs substrate, it should be noted that each of the multilayer reflection structures up and below the active layer and corresponding to the foregoing first and second multilayer reflection structures, are formed of an alternate repetition of an AlGaAs layer and a GaAs layer. As a result of the use of the AlGaAs layer and the GaAs layer in the first and second multilayer reflection structures, a large refractive index is secured between the first and second layers or between the third and fourth layers noted previously, and the number of stacks needed for the effective performance of the reflective structure can be reduced, from about forty for the case of constructing the laser diode on an InP substrate, to about twenty.
However, in view of the large wavelength of the optical radiation in the 1.3 .mu.m band, the thickness of each of the first and second or third and fourth layers is increased inevitably as compared with a conventional laser diode operable in the 0.8 .mu.m band, and the problem of increase of total thickness of the multilayer reflection structure is not entirely resolved. In a multilayer reflection structure, it should be noted that each of the layers has to have a thickness corresponding to one-quarter the wavelength (.lambda./4) of the optical radiation to be reflected as noted previously.